Multi-mode variable rate digital cable receiver

ABSTRACT

Carrier signals modulated by information (video and/or data) signals are received through a cable and are converted to modulated signals at an intermediate frequency. The IF signals are sampled at a particular frequency to produce digital information signals. The digital information signals are introduced to a variable interpolator which produces first digital signals. The first digital signals are introduced to a complex multiplier which produces second digital signals. The second digital signals pass to an adaptive equalizer which selects for each of the second signals in accordance with the amplitude of such second signals, an individual one of a multitude of amplitude levels involved in quadrature amplitude modulation. These selected amplitude levels represent the information (video and/or data). The output signals from the adaptive equalizer are introduced to a first signal recovery loop which includes a first numerically controlled oscillator. The oscillator operates upon the variable interpolator to obtain the production by the variable interpolator of the first digital signals in the correct subinterval of the time period that each of the digital information signals is produced. The output signals from the adaptive equalizer are also introduced to a second signal recovery loop which includes a second numerically controlled oscillator. This oscillator operates upon the complex multiplier to maintain the frequency of the second digital signals at the frequency of the digital information signals.

This application is a continuation of patent application Ser. No09/078,102 filed May 13, 1998.

This invention relates to a system for, and method of receivinginformation (e.g., video and/or data) signals transmitted through acable from a plurality of television stations each operative in anindividual frequency range and for recovering the informationrepresented by the information signals.

BACKGROUND OF THE INVENTION

Systems have been in existence for a number of years for receivingsignals from a plurality of television stations and for transmittingthese signals through a cable to a subscriber. Each of the televisionstations provides signals in an individual range of frequencies. Forexample, the signals from the different television stations may havedifferent frequencies in a range between approximately fifty megahertz(50 MHz) to approximately eight hundred and fifty megahertz (850 MHz).The signals from the different television stations in the frequencyrange of approximately 50-850 MHz modulate a carrier signal having asuitable carrier frequency.

The television receivers then convert the carrier signals to signals atan intermediate frequency such as approximately forty-four megahertz (44MHz). These intermediate frequency (IF) signals are the n demodulated atthe television receivers and the demodulated signals are processed torecover the data signals from the individual ones of the televisionstations. The processing of the signals occurs on an analog basis.

It is well recognized that the processing of the signals on an analogbasis to recover the information in the information signals is not asprecise as would ordinarily be desired. The recovery of such informationon a precise basis by analog techniques is especially difficult in viewof the fact that the information signals are encoded using quadratureamplitude modulation (QAM) with a multitude of amplitude levels.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment of the invention, carrier signals modulated byinformation (video and/or data) signals are received through a cable andare converted to modulated signals at an intermediate frequency. The IFsignals are sampled at a particular frequency to produce digitalinformation signals. The digital information signals are introduced to avariable interpolator which produces first digital signals. The firstdigital signals are introduced to a complex multiplier which producessecond digital signals. The second digital signals pass to an adaptiveequalizer which selects, for each of the second signals in accordancewith the amplitude of such second signals, an individual one of amultitude of amplitude levels involved in quadrature amplitudemodulation. These selected amplitude levels represent the information(video and/or data).

The output signals from the adaptive equalizer are introduced to a firstsignal recovery loop which includes a first numerically controlledoscillator. The oscillator operates upon the variable interpolator toobtain the production by the variable interpolator of the first digitalsignals in the correct subinterval of the time period that each of thedigital information signals is produced. The output signals from theadaptive equalizer are also introduced to a second signal recovery loopwhich includes a second numerically controlled oscillator. Thisoscillator operates upon the complex multiplier to maintain thefrequency of the second digital signals at the frequency of the digitalinformation signals.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a block diagram of a prior art system, including a quadratureamplitude modulation receiver, for receiving television signals througha cable from a plurality of television channels and for converting thetelevision signals for each channel to video and audio;

FIG. 2 is a block diagram of a quadrature amplitude modulated televisionreceiver used in the prior art as the receiver of FIG. 1;

FIGS. 3a-3 b are a block diagram of a quadrature amplitude modulatedreceiver constituting one embodiment of the invention;

FIGS. 4a-4 b are a block diagram of a quadrature amplitude modulatedreceiver constituting a second embodiment of the invention; and

FIG. 5 is a block diagram of an arrangement formed by a plurality offilters for providing a selection between adjacent television channelsand shows the signals produced by each of the adjacent channels.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of a system, generally indicated at 10 andincluding a quadrature amplitude modulation (QAM) receiver, forreceiving television signals through a cable from a plurality oftelevision channels and for converting the television signals for eachchannel to video and audio. The system 10 is well known in the priorart. The system 10 includes a tuner 12 for selecting, for a particularone of the channels or stations, the carrier signals modulated byinformation signals representing video and audio.

The signals from the tuner 12 are introduced to a surface acousticfilter wave (SAW) 14 which acts as a band pass filter to pass thesignals within a particular intermediate frequency. The signals from thefilter 14 then pass to an automatic gain control (AGC) stage 16 forregulating the power of the intermediate frequency signals. The signalsare then introduced to a quadrature amplitude modulation receiver 18.The receiver 18 provides individual ones of a plurality (generally inthe hundreds—e.g., 256) of amplitude and phase levels to represent thereceived information. The receiver 18 is able to provide individual onesof as many as 256 different amplitude and phase levels because thesignals passing through the cable 11 are relatively strong, particularlyin comparison to the signals passing from satellites. Feedback signalsare introduced on a line 20 from the receiver 18 to the stage 16 toregulate the gain of the stage.

The video signals from the receiver 18 pass to a decompressor 22 whichdecompresses the received signals. The decompressor 22 may be an MPEGdecompressor which is well known in the prior art. The decompressedsignals pass to a video graphics display 24 which may be a conventionalanalog TV set.

The signals from the receiver 18 are also introduced to an audiodecompressor 26 which may be an MPEG decompressor which is well known inthe prior art. The signals from the decompressor 26 pass to adigital-to-analog converter 28. The output from the converter 28provides the audio information. The signals from the converter 28 andfrom the decompressor 22 are introduced to an RF modulator 30. The videois provided from the output from the modulator 30 to a conventional TVset.

FIG. 2 illustrates in block form a traditional (prior art) receiverarchitecture, generally indicated at 32, for receiving RF signalsthrough the cable 11. The signals in the cable 11 are introduced to atuner 36 which is shown within broken lines in FIG. 2. The tuner 36includes a down convert stage 38 and surface acoustic wave filter (SAW)40. The stage 38 converts the signals at the carrier frequency tosignals at an intermediate frequency such as forty-four megahertz (44MHz) or thirty-six megahertz (36 MHz). The signals from the surfaceacoustic wave (SAW) 40 pass to an automatic gain control (AGC) stage 42.

A pair of multipliers 44 and 46 receives the gain control signals fromthe stage 42. The multiplier 44 also receives signals having the samefrequency as the frequency of the signals from the stage 42 and having aphase of zero degrees (0°). This is indicated at 48. The multiplier 46also receives signals having the same frequency as the frequency of thesignals from the stage 42 and having a phase of 90°. This is indicatedat 50 in FIG. 2. Low pass filters 52 and 54 respectively limit thefrequency of the signals from the stages 44 and 46.

Analog-to-digital converters 56 and 58 respectively convert the analogsignals from the filters 52 and 54 to digital signals. The signals fromthe converters 56 and 58 are introduced to an automatic gain controlloop 57 which operates in a conventional manner to regulate the gain ofthe signals from the stage 42.

The digital signals pass to filters 60 and 62 which may constitutesuitable low pass filters such as Nyquist filters. The signals from thefilters 60 and 62 are introduced to an adaptive equalizer 64. Anadaptive equalizer suitable for use as the equalizer 64 is disclosed indetail in co-pending application Ser. No. 08/285,504 filed by HenrySamueli and Charles P. Reames on Aug. 3, 1997, for a “System for, andMethod of, Processing Quadrature Amplitude Modulated Signals” andassigned of record to the assignee of record of this application.

The adaptive equalizer 64 may include a feed forward equalizer, adecision feedback equalizer and a slicer associated with theseequalizers and providing amplitude approximations of increasingsensitivity at progressive times. In this way, the adaptive equalizer 64selects, for each individual one of the signals introduced to theadaptive equalizer, an individual one of many (e.g., 16) of amplitudelevels closest to the amplitude of such individual one of the signalsintroduced to the adaptive equalizer. The output from the adaptiveequalizer 64 is introduced to a forward error correction (FEC) stage 66which provides corrections to discovered errors in a manner well knownin the art. The output from the forward error correction stage 66represents the MPEG compressed television information.

The output from the adaptive equalizer 64 passes to a symbol recoveryloop 70. The loop 70 operates on an analog basis and includes a firstvoltage controlled oscillator (not shown). The first voltage controlledoscillator produces a clock signal which is introduced to the converters56 and 58 to adjust and regulate the frequency at which the analogsignals from the filters 52 and 54 are converted to digital signals.

The output from the adaptive equalizer 64 also passes to a carrier loop72. The loop 72 operates on an analog basis and includes a secondvoltage controlled oscillator. The second voltage controlled oscillatorproduces a sinusoidal signal which is introduced to the stage 48 toregulate the frequency at which the stage 48 is operated. In this way,the frequency of operation of the stage 48 is regulated to conform tothe frequency of the signals that are introduced to the multiplier 44from the automatic gain control stage 42. As will be appreciated, thefrequency of the signals in the stage 50 conforms to the frequency ofthe signals in the stage 48, but has a phase separated by 90° from thephase of the stage 48.

FIGS. 3a-3 b illustrates one embodiment, generally indicated at 80, ofan invention constituting one embodiment of the invention for providinga multi-mode variable rate digital cable receiver. The embodiment 80includes the digital cable 11, the tuner 36 and the automatic gaincontrol stage 42 also shown in FIG. 2. The signals from the stage 42 areintroduced to a multiplier 82 as are the signals from a fixed oscillator84 having a suitable frequency such as approximately 35.2 MHz or 28.8MHz.

The signals from the multiplier 82 are introduced to a low pass filter86. When the fixed oscillator 82 has a frequency of 28.8 MHz, the AGCstage 42 may have a center frequency of 36 MHz and the filter 86 maypass a range of frequencies to approximately 7.2 MHz. Similarly, whenthe fixed oscillator 82 has a frequency of 35.2 MHz, the AGC stage 42may have a center frequency of 44 MHz and the filter 86 may pass a rangeof frequencies to approximately 8.8 MHz.

The signals from the filter 86 are introduced to an analog-to-digitalconverter 88 which also receives signals from a fixed oscillator in theform of a phase lock loop clock generator stage 90. The stage 90 mayprovide a signal at substantially the same frequency as the frequency ofthe signals from the AGC stage 42. An AGC loop 91 corresponding to theAGC loop 57 in FIG. 2 may be provided between the converter 88 and theAGC stage 42 to regulate the gain of the signals in the AGC stage.

The signals from the converter 88 are introduced to a pair ofmultipliers 92 and 94. The multiplier 92 also receives in-phase (orcosine) signals on a line 96 at the same frequency as the signals fromthe converter 88. The multiplier 94 also receives quadrature (or sine)signals on a line 98 at the same frequency as the signals from theconverter 88. The signals from the multiplier 92 are introduced tofilter stages 100, 102 and 104 respectively designated as “halfband,”“quarterband” and “eighthband.” In like manner, the signals from themultiplier 94 are introduced to filter stages 106, 108 and 110respectively designated as “halfband,” “quarterband” and “eighthband.”The signals from the stages 106, 108 and 110 respectively have frequencyranges one half (½), one fourth (¼) and one eighth (⅛) of the frequencyrange of the signals from the multiplier 92.

The signals from selected one of the filters 100, 102 and 104 pass to avariable interpolator 112 as do the signals from a selected one of thefilters 106, 108 and 110. The output from the variable interpolator 112in turn passes to low pass filters 114 and 116. The filters 114 and 116may constitute Nyquist filters which are well known in the art.

The outputs from the filters 114 and 116 are introduced to a complexmultiplier 118. The complex multiplier 118 may be constructed in amanner similar to that disclosed in detail in co-pending applicationSer. No. 09/013,964 filed in the United States Patent Office on Jan. 27,1998 in the names of Henry Samueli, Alan Y. Kwentus and Thomas Kwon asjoint inventors for a “Multi-Mode Variable Rate Digital SatelliteReceiver” and assigned of record to the assignee of record of thisapplication.

Connections are made from the outputs of the complex multiplier 118 tothe inputs of an adaptive equalizer 120 corresponding in construction tothe adaptive equalizer 64 shown in FIG. 2. The outputs from the adaptiveequalizer 120 are introduced to a forward error correction stage (FEC)stage 122 corresponding to the stage 66 in FIG. 2. The output from thestage 122 represents the MPEG compressed television information.

The outputs from the adaptive equalizer 120 are connected to a carrierrecovery loop 124 which operates on a digital basis. The carrierrecovery loop 124 may include a phase detector for detecting phaseerrors and may also include a loop filter. The output from the carrierrecovery loop 124 passes to a quadrature direct digital frequencysynthesizer (QDDFS) 126 which may be a numerically controlled oscillatorproviding two (2) output signals (e.g. cosine and sine) separated inphase by 90° from each other. The QDDFS 126 introduces the inphase (orcosine) and the quadrature (or sine) signals to the complex multiplier118.

The signals from the adaptive equalizer 120 also pass to a symbolrecovery loop 128 which operates on a digital basis. The symbol recoveryloop 128 may include a phase detector and a loop filter as in thecarrier recovery loop 124. The symbol recovery loop 128 may also includea numerically controlled oscillator. The output from the numericallycontrolled oscillator is introduced to the variable interpolator 112.

The tuner 36, the multiplier 82 and the low pass filter 86 operate toreduce the frequency of the RF signals passing through the cable 11. Theanalog-to-digital converter stage 88 oversamples the analog signals fromthe low pass filter 86 in converting the analog signals to digitalsignals. The digital signals are then converted to in-phase signals inthe multiplier 92 and to quadrature signals in the multiplier 94.

The signals from the multiplier 92 then pass through a selective one ofthe filters 100, 102 and 104 depending upon the rate at which thesignals are being produced. For example, the half band filter 100 passesthe signals when the symbol rate is approximately seven (7) megabaud.The quarterband filter 102 passes the signals when the symbol rate isbetween approximately 3.5 megabaud and 7 megabaud. The eighth bandfilter 104 passes the signals when the symbol rate is betweenapproximately 1.75 megabaud and 3.5 megabaud.

FIG. 5 shows the half band filter 100, the quarter band filter 102 andthe eighth band filter 104. FIG. 5 also includes a curve 140 showing therange of frequencies passed by the surface acoustic wave filter 40, acurve 142 showing the range of symbol frequencies passed by the halfband filter 100, a curve 144 showing the range of symbol frequenciespassed by the quarter band filter 102 and a curve 146 showing the rangeof symbol frequencies passed by the eighth band filter 104. FIG. 5 alsoshows by darkened areas the range of frequencies of adjacent channelsfor each individual one of the half band filter 100, the quarter bandfilter 102 and the eighth band filter 104. By way of illustration, theband of frequencies for a channel when using the half band filter 100may correspond to the range of frequencies of the darkened area in thetop curve in FIG. 5.

The variable interpolator 112 in FIG. 3b operates on the selective oneof the filters 100, 102 and 104 and the selective one of the filters106, 108, and 110 to pass the signals from the selective ones of thesefilters. The symbol recovery loop 128 operates digitally to regulate theoperation of the variable interpolator 112 in passing the signals fromthe selective ones of the filters 100, 102 and 104 and the selective oneof filters 106, 108 and 110 in the correct subinterval of the symbolperiods that the digital signals are produced by the converter 88. Aspreviously disclosed, the symbol recovery loop 128 includes anumerically controlled oscillator for operating upon the signals fromthe adaptive equalizer 120 to produce error signals for regulating thephase of the signals passing through the variable interpolator 112. Thisregulation provides for the passage of these signals through thevariable interpolator 112 in the correct subinterval of the symbolperiods that the digital signals are produced by the converter 88.

The complex multiplier 118 operates on a digital basis to translate thefrequency of the digital data signals to a zero carrier frequency. Aspreviously disclosed, the carrier recovery loop 124 includes thequadrature direct digital frequency synthesizer (QDDFS) 126 foroperating upon the signals from the adaptive equalizer 120 to produceerror signals for regulating the frequency of the signals passingthrough the complex multiplier 118.

In the system shown in FIG. 3b, the variable interpolator 112 isdisposed in front of the complex multiplier 118 in the progression ofstages in the system. This is in contrast to the relative dispositionsof the variable interpolator and the complex multiplier in the satellitereceiver disclosed and claimed in co-pending application filed Jan. 27,1998 in the names of Henry Samueli, Alan Y. Kwentus and Thomas Kwon asjoint inventors for a “Multi-Mode Variable Rate Digital SatelliteReceiver” and assigned of record to the assignee of record of thisapplication.

The disposition of the variable interpolator 112 in front of theadaptive equalizer 120 offers certain advantages in the system of theinvention when used to receive the television signals through a cable.One of the advantages of disposing the variable integrator 112 in frontof the complex multiplier 118 in the system of this invention resultsfrom the fact that the carrier frequency error of the signals passingthrough the cable is relatively low. Because of this, the variableinterpolator 112 and the symbol recovery loop 128 are able to regulatethe frequency of the signals passing from the converter 88 quite closelybefore the signals are introduced to the complex multiplier 118.

The complex multiplier 118 and the carrier recovery loop 124 are thenable to provide a further and more refined regulation in the frequencyof the signals. In this way, the frequency of the signals can beprecisely regulated to correspond to the frequency at which the digitalsignals are produced in the converter 88.

FIGS. 4a-4 b show shows a system which is similar in many respects tothe system shown in FIGS. 3a-3 b. However, in the system shown in FIGS.4a-4 b the signals from the automatic gain control stage 42 areintroduced directly to the analog-to-digital converter 88. Thiseliminates several stages from the embodiment shown in FIGS. 3a-3 b.

A variable interpolator for use as the variable interpolator 112 isknown in the prior art. The variable interpolator 112 may be constructedin accordance with the disclosure in any of the following publications:

Gardner, Floyd M., “Interpolator in Digital Modems-Part 1:Fundamentals”, IEEE Transactions on Communications, No. Mar. 3, 1993.

Harris, Fred, “On the Relationship Between Multirate Polyphase FIRFilters and Windowed, Overlapped, FFT Processing”, Proceedings of theTwenty Third Asilomar Conference on Signals, Systems and Computers, Oct.30-Nov. 1, 1989.

Harris, Fred, et al. “Modified Polyphase Filter Structure for ComputingInterpolated Data As Successive Differential Corrections”, Proceedingsof the 1991 International Symposium on Circuits and Systems, Singapore,Jun. 11-14, 1991.

Crochiere, Ronald E, and Rabiner, Lawrence R., Multirate Digital SignalProcessing, Englewood Cliff, N.J.: Prentice Hall, 1983.

U.S. Pat. No. 5,504,785-Apr. 2, 1996-Digital Receiver for VariableSymbol Rate Communications.

What is claimed is:
 1. A method of receiving information signals, eachinformation signal operative in an individual frequency range, andrecovering information represented by the information signalscomprising: receiving carrier signals modulated by the informationsignals; reducing the frequency of the carrier signals to a particularintermediate frequency to obtain IF signals; sampling the IF signals toobtain digital information signals; band select filtering the digitalinformation signals to obtain adjacent channel filtered digital datasignals; variable interpolating the adjacent channel filtered digitaldata signals to obtain first signals; complex multiplying the firstsignals to provide second signals, baseband adaptive equalizing thesecond signals to select, for each of the second signals, an individualone of a plurality of amplitude levels in quadrature amplitudemodulation; feedback operating upon the variable interpolating, inresponse to the baseband adaptive equalizing, to provide a sampling ofthe digital information signals for obtaining the first signals in themiddle of the period that each of such digital information signals isproduced; and feedback operating upon the complex multiplying, inresponse to the basedband adaptive equalizing, to maintain the frequencyof the second signals at the frequency of the digital informationsignals.
 2. The method as set forth in claim 1, wherein the feedbackoperating upon the variable interpolating includes varying the frequencyof signals from the baseband adaptive equalizing to provide the samplingof the digital information signal for obtaining the first signals in thecorrect subinterval of the period that each of such digital informationsignals is produced.
 3. The method as set forth in claim 1, wherein theoperating upon the complex multiplier includes varying the frequency ofthe signals from the baseband adaptive equalizing to maintain thefrequency of the second signals at the frequency of the digitalinformation signals.
 4. The method as set forth in claim 1, furthercomprising feedback regulating, in response to the digital informationsignals, the gain of the IF signals before sampling the IF signals. 5.The method as set forth in claim 1, further comprising reproducing, inresponse to the amplitude levels from the baseband adaptive equalizing,the information represented by the carrier signals modulated by theinformation signals.